You're already speaking to your reader through your book.
Even so, it is sometimes helpful to have a way to break the fourth wall and speak to them about your book.
Director's commentary is a visually flexible option for doing so, allowing you to overlay a more casual tone, add clarifications or caveats, anticipate niche objections, or talk about the book itself.
Make Time by Jake Knapp and John Zeratsky, using director's commentary to overlay a different tone, shifting from "two authors giving joint advice" to "one author sharing individual experiences".
How it helps
This is a flexible, situational pattern can bracket niche objections and signpost skippability to create parallel reader experiences.
It's a natural format for a breadcrumb author bio, and a good fit for providing additional reassurance, encouragement, emphasis, or reminders to a reader (similar in function to messy manual markup).
For example, if your book is asking readers to go out into the world and take action, it's likely that some of those requests will feel disproportionately hard, scary, or intimidating. In those cases, you might choose to use director's commentary to speak directly to the reader, reassuring them that it's going to be okay, reminding them that it's worth it, or relating to them by sharing a vulnerable example of how you felt while doing the same thing for the first time.
Understanding Comics by Scott McCloud as an extreme examples of director's commentary, where the author's avatar uses a more conversational style of commentary compared to the book's default, more academic prose.
How to make it work
Director's commentary is stylistically flexible.
The simplest implementation is "Note from the author" in a footnote or sidenote.
More visually distinctive options are also viable (like an inline callout, with or without the author's face).
One important consideration: a human face is extremely attention-grabbing. If you decorate your director's commentary with a picture of your face, you're essentially declaring that as he most important element on the page. If your director's commentary is meant to be more of an optional aside, then consider skipping the avatar and using subtler typography and placement to makes it subservient to your core prose.